The membrane 2 therefore allows the anodic compartment 5 and the cathodic compartment 6 to be separated. The catalytic layers 3, 4 generally comprise nanoparticles supported by carbon aggregates. Gas diffusion layers 7, 8 (carbon fabric, felt, etc.) are placed on either side of the MEA 1, in order to ensure electrical conduction, uniform distribution of the reactive gases, and evacuation of water produced by the reaction. A system of channels 9, 10 placed on either side of the MEA delivers the reactive gases and evacuates, to the exterior, water and excess gases.
At the anode 3, decomposition of hydrogen absorbed on the catalyst produces protons H+ and electrons e−. The protons then pass through the polymer membrane before reacting with oxygen at the cathode 4.
Reaction of the protons with oxygen at the cathode leads to the formation of water and to the production of heat 4.
Depending on the production method of the hydrogen, the gas may comprise impurities. It has been demonstrated that carbon monoxide and sulfur compounds are particularly detrimental to cell operation.
In this context, maximum concentration thresholds have been proposed to standardize the quality of the hydrogen used in fuel cells in automotive applications: for example 0.2 μmol/mol for CO and 0.004 μmol/mol for sulfur compounds.
Hydrogen sulphide (H2S) is, in general, the main sulfur compound present in the hydrogen. The generally accepted dissociative absorption mechanism of H2S on platinum, as described in the article by M.-V. Mathieu, M. Primet, Applied Catalysis, 9 (3) 361-370 (1984), is the following:H2S+Pt→Pt−S+H2  (1)H2S+Pt−H→Pt−S+ 3/2H2  (2)
Sulfur adsorption on the platinum (Pt—S) poisons catalytic sites and this adsorbed sulfur is particularly difficult to desorb (very strong Pt—S chemical bond). The sulfur blocks catalytic sites for the oxidation reaction of the hydrogen, thereby leading to an increase in the anodic overvoltage by a few tens of millivolts and a decrease in the performance of the cell.
Whereas carbon monoxide desorbs easily in the presence of pure hydrogen, sulfur adsorption on platinum is considered to be partially reversible or even irreversible in the presence of pure hydrogen: the sulfur remains bonded to the platinum and the electrical performance of the cell remains poor. It should be noted that pollution by air-side sulfur compounds also leads to a partially reversible decrease in the performance of the cell.
A number of methods have been proposed to “clean” the surface of the catalyst in case of poisoning by species highly absorbed on the surface of the catalyst, and thus to improve the electrical performance of the fuel cell, in order to improve the performance of a cell stack after a phase of pollution with sulfur compounds.
1) A first method uses cyclic voltammetry:
It has thus been proposed to clean the surface of the catalyst by carrying out one or more cyclic voltammetry cycles on the anode as described in: F. H. Garzon, T. Rockward, I. G. Urdampilleta, E. L. Brosha, F. A. Uribe, ECS Trans., 3 (1) 695-703 (2006). This cyclic voltammetry allows a sufficiently high anodic potential (typically a potential comprised between 0.9V and 1.3V) to be reached to oxidize the sulfur, one of the possible reactions of which is:Pt−S+4H2OSO42−+8H++6e−Pt  (3)
Nevertheless, this method requires the use of an external generator to be able to apply the potential sweep. Such a generator is not necessarily present in real systems.
2) A second known prior-art method uses power pulsing:
This solution consists in applying a short pulse in order to reach the potential required for oxidation of the sulfur layer (3). This solution was recently patented by Los Alamos National laboratory and is notably described in the article: F. A. Uribe, T. Q. T. Rockward, Cleaning (de-poisoning) PEMFC electrodes from strongly adsorbed species on the catalyst surface. US 2006/0249399 A1. 2006. This pulse may last 20 seconds, at 30 A and 1.4V. In this solution, the inventors apply a potentiostatic mode, i.e. the voltage of the cell is fixed and the current density changes over time. They propose:                either to stop hydrogen flow completely, while maintaining potential difference, with the aim of oxidizing residual hydrogen, before disconnecting the fuel cell and applying the pulse in order to oxidize the sulfur compounds;        or to decrease the hydrogen flow and apply a larger pulse in order to oxidize the hydrogen and the sulfur compounds.        
Just like the preceding solution, this solution requires the use of an external power source. This solution is presented for the case of potentiostatic operation.
3) A third solution uses a period at OCV (open-circuit voltage=zero current):
It has also been proposed to place the cell at OCV, i.e. to stop the supply of the cell in order to recover its performance after poisoning by hydrogen sulphide, as described in the articles I. G. Urdampilleta, F. A. Uribe, T. Rockward, E. L. Brosha, B. S. Pivovar, F. H. Garzon, ECS Trans., 11 (1) 831-842 (2007) and D. Imamura, Y. Hashimasa, ECS Trans., 11 (1) 853-862 (2007). The H2S poisoning is followed by a return phase under pure hydrogen, then a phase at the OCV for several hours in order to allow performance recovery. The several-hour-long period at OCV seems to allow sulfur compounds to be desorbed. This solution corresponds to galvanostatic operation. Poisoning occurs at fixed current, the potentials of the electrodes and the voltage of the cell are therefore allowed to vary.
4) A fourth solution consists in regenerating the anode by decreasing fuel flow and therefore decreasing the stoichiometric coefficient so as to enable the polluting species to be oxidized. Such a solution has especially been described in patent application US 2010/0233554.
Generally, poisoning of an electrode by compounds, for example sulfur compounds, leads to formation, on the surface of the catalyst, of a sulfur layer that can be oxidized, by one of the possible reactions, such as reaction (3), only at a high electrode potential.
On the anode side, the overall oxidation reaction of the hydrogen is the following:H2→2H++2e−  (4)
One hydrogen molecule therefore produces 2 protons and 2 electrons. The hydrogen flow required to establish a current I (in A) is given by the following equation:
                                          Q                          H              2                                ⁡                      (                          mol              ⁢                                                          ⁢                              s                                  -                  1                                                      )                          =                  I                      2            ×            F                                              (        5        )            
where F is the Faraday constant (in A·s·mol−1).
In fuel-cell systems, hydrogen, at the anode, and air (or oxygen), at the cathode, are very often injected in excess (operation in superstoichiometric mode). The hydrogen flow required to establish the current is therefore multiplied by the stoichiometric coefficient StH2:
                                          Q                          H              2                                ⁡                      (                          mol              ⁢                                                          ⁢                              s                                  -                  1                                                      )                          =                              St                          H              2                                ×                      I                          2              ×              F                                                          (        6        )            
the stoichiometric coefficient corresponding to the excess of injected gas.
For a constant current density, if the hydrogen stoichiometric coefficient is decreased below unity the hydrogen flow will no longer be sufficient to supply the amount of electrons necessary to keep the current at its setpoint value. Thus, in a first step, the current is maintained by oxidation of residual hydrogen. Next, as there is an insufficient amount of hydrogen, anodic potential will increase. In order to maintain constant current, reaction mechanisms other than the oxidation reaction of hydrogen are solicited. These other mechanisms, for example reaction (3), which is the oxidation of sulfur adsorbed on the surface of the platinum to form sulfuric acid, are made possible by virtue of the increase in potential, leading to regeneration of the anode which was previously polluted.
Nevertheless, the Applicant has observed that during the anode regeneration period operation at the cathode degrades, the cathode potential dropping, possibly because of cathode-corrosion effects especially caused by the overabundance of water produced in the regeneration phase.